Alloys resistant to high temperatures



United States Patent 3,265,491 ALLOYS RESISTANT TO HIGH TEMPERATURES William T. Kaarlela, Fort Worth, Tex., assignor to General Dynamics Corporation, Fort Worth, Tex., a corporation of Delaware N0 Drawing. Filed Apr. 15, 1965, Ser. No. 448,249 5 Claims. (Cl. 75134) This application is a continuation-in-part of .applicants co-pending application, Serial No. 261,887, filed February 28, 1963, now abandoned.

The invention relates in general to refractory metal alloys capable of resisting high temperatures and more particularly, -to alloys of such character as may be well suited for the brazing of metals such as colum'bium' and molybdenum and their alloys and having additional utility as high temperature protective coatings and as structural alloys.

It has become increasingly important, particularly in missile, spacecraft and aircraft applications, to use materials which are capable of withstanding extremely high temperatures. In addition to molybdenum, one such material is columbium, which is a refractory metal possessing a melting point in the vicinity of 4400 F. From a design standpoint, it is an excellent material because of its high strength to weight ratio in the 2000 to 2500 F. range of service temperatures. It does not strain-harden rapidly, allowing cold working up to 99% without annealing, and it is therefore particularly suitable (for the forming of parts of complex shape. Columbium is also characterized by moderate density (comparable to iron and nickel), and a high melting point, with good strength retention above the useful range of currently available alloys.

Useful though they are in high temperature areas, joinder of these metals and their alloys presents problems. Thus, although both columbium and molybdenum may be welded, nitrogen contamination resultant therefrom is a deleterious obstacle since it causes an increase in the tendency for crater cracking, serious loss of ductility and an increase in the transition temperature. Welding also presents the problem of loss of strength due to recrystallization. In the handling of high strength columbium alloys, recrystallization occurs between 2200 and 2800 F., depending upon the alloy makeup. Welding involves these high temperatures and where recrystallization as a result occurs, a loss of approximately 50% in tensile strength may be anticipated.

However, it has been found that by using the alloys of this invention, excellent joinder of these metals is effected and a high temperature resistant joint is produced which is compatible with the strength characteristics of the joined materials. Vacuum environment or inert atmospheres such as argon gas are utilized to prevent oxidation of the columbium or molybdenum and their alloys during the brazing, only moderate efforts being necessary for purification of the argon gas preparatory to brazing.

The alloys of this invention are additionally useful as coatings for columbium and for other materials characterized by low oxidation resistance at elevated temperatures, i.e., of approximately 2000 F.

Further utility for these alloys is found in structural applications, such as castings, and in other areas of metal forming where adequate oxidation resistance at elevated temperatures is difiicult to achieve and maintain and where both structural and non-structural provisions are a requirement.

Accordingly, it is an object of this invention to provide alloys well suited for the brazing of refractory metals such as columbium and molybdenum and their alloys; which alloys are capable of providing excellent joint strength at elevated temperatures.

It is another object of this invention to provide alloys of the character described which do not require excessively high temperatures for brazing and which possess adequate ductility.

A further object is to provide alloys, as described, which do not cause excessive erosion of the base alloys when applied thereto in brazing applications.

Yet a further object is to provide alloys suitable as protective coatings for preventing oxidation of materials having a high susceptibility thereto at elevated temperatures.

Another object is the provision of alloys adapted to casting applications which call for materials possessing adequate oxidation resistance at elevated temperatures.

These and other objects and advantages of this invention will become apparent to those skilled in the art from the following description of the alloys and their characteristics and the claims directed thereto.

In general, the alloys of this invention include as matrix elements titanium and chromium in the percentage-bywei-ght ranges indicated. Each of these matrix elements is characterized by its compatibility with columbium, molybdenum and the alloys thereof. At least one alloying element selected from the group consisting of zirconium and tin, and proportioned as set forth, is added to the matrix elements. The resulting alloys have produced brazed joints possessing excellent joint strength at high temperatures. This brazing has been effected without the use of extremely high temperatures. Abilityof the alloys to withstand high temperatures with a minimum of deterioration has contributed to their further utilization as coatings and as barriers for protection against oxidation of various materials inherently susceptible thereto. Formability at practicable temperatures and the aforementioned excellent high temperature characteristics further dictate use of the alloys in the area of structural material, particularly in casting applications.

It is to be understood that percentages used 'herein in both specification and claims, to describe ingredient proportions, are percentages by weight unless otherwise stated.

Chromium, as an elemental constituent to each of the alloys has been found to be quite compatible with columbium, forming a difliusi-on layer which is much like that formed by titanium, but which shows more interaction at the interface with the brazing alloy. As employed herein, the general range of chromium is from about 20% to about 60% by weight of the alloy. A preferred range has been found to be from about 30% to about 50%. Superior alloy compositions have been established incorporating the specific proportions of chromium tabulated below.

Titanium, like chromium, serves as a matrix element in certain of the present alloys. Like chromium, it has been found to be quite compatible with columbium and forms a narrow alloy layer. It enters into the alloys of this invention in the general range of from about 20% to about 60%. Its preferred range is from about 30% to about 50%. Superior alloy compositions include titanium in the quantities set forth hereinafter.

The addition of tin or zirconium serves to depress the melting temperature of the matrix alloy. Both of these elements show limited solid solubility in chromium, form a chromium rich eutectic and are therefor useful for control of the alloy melting point. Tin also serves to improve fluidity and wetting. Zirconium increases the toughness of the alloy. The general range for zirconium and tin is from about 1% to about 20%; the preferred range being from about 3% to about 15%. Additions of these elements are to be made judiciously, since their presence in quantities greater than the upper limits of their respective general ranges will result in excessive dissolution of columbium at those temperatures required for brazing.

Further, each alloy was tested for structural integrity, brazing characteristics and environmental resistance. such values as brazing temperature, re-melt temperature, shear strength, oxidation resistance, toughness factor and brazeability on columbium and molybdenum were estab- Although the process for alloy formulation is subject lished and are set forth in tabular form below:

1 Value of 1000 or higher indicates good toughness.

to considerable variation, the alloys of the present invention have, for test purposes, been formulated by mixing the elemental ingredients together in the desired proportions in powder form. The mixture is subsequently briquetted into a compact; then melted in a cold-hearth, water-cooled, copper crucible. If ductile, the alloy is then rolled to form a foil, or in the alternative, broken and crushed into a powder to be used in this form.

As formulated herein, the alloys of this invention have taken the powder form. Application is effected by mixing the powder alloy with polyvinyl alcohol in a slurry, which is then painted on the joint to be brazed. The alloy is then heated to a temperature above its melting point in a protective environment, such as in a vacuum or using an argon atmosphere or other suitable inert gases, for protection against the detrimental effect of oxidation. As hereinabove stated, moderate efforts should be made to purify the argon gas, if best results are to be obtained. This has been accomplished satisfactorily, in the present instance, by passing the argon through an activated alumina dryer, a 100 F. Dry-Ice-acetone cold trap, and a closed zirconia tube filled with titanium strips and operating at 1750" F. The protection offered by the inert argon gas is important. Should atmospheric contamination occur it will be directly reflected in reduced flow and Wetting of the brazing alloy, resulting in an inferior joint.

For purposes of the tests, the results of which are reflected in the tabulations below, brazed lap shear test specimens were made up using a columbium alloy incorporating by weight 10% titanium and 10% molybdenum. These were employed as the members to be joined and lap shear tested. Time at temperature prior to testing was l-S minutes. Employing an A-frame type lap shear tension test apparatus, failure was made to occur within one minute by steadily increasing the mechanical stress upon the specimen by means of a floating screw. Test temperatures were effected by the induction heating of a graphite susceptor surrounding the specimen. Stress was measured by means of a calibrated load link in conjunction with a strain recorder. Specimens were confined under a protective, substantially inert, argon atmosphere during heating, testing and cooling.

Each of the alloys of this invention is set forth in the table below with an indication both as to the general range of its ingredients and as to the specific composition of the particular alloy or alloys tested:

During these tests, a remelt temperature rise phenomenon was observed in connection with the two alloys designated number 1 and 2 in the above tabulation of test results. That is to say the melting temperature of these particular brazing alloys ranged between 2390 F. and 2440 F. However, in tests after brazing of the columbium lap shear specimen, it was found that these alloys did not remelt at 3000 F., the highest temperature checked.

Numerous high temperature oxidation studies of the above alloys have been conducted. Oxidation tests were carried out in the following manner: Small specimens of the alloys were are melted and shaped into the form of smooth, round buttons. These buttons were weighed to the nearest 0.1 milligram then exposed to a heated air environment of 2000 F. in 8 hr. increments until a total exposure time of hours had been accumulated. The specimens were weighed at the end of each 8 hr. exposure period and the percentage weight change noted and recorded. These exposure tests were conducted in a stillair filled resistance heated furnace. The specimens were placed on non-reactive beryllium boats for the heat exposure tests.

Visual and microstructural studies indicate the usefulness of these alloys as protective coatings for materials having low resistance to oxidation at elevated temperatures.

Application of the alloys may be accomplished by painting or spraying the alloy in powdered form on the surface of the metal to be protected and then heating the coated assembly until the powdered alloy melts to form a continuous surface coating.

It is the above qualities of the alloys of this invention which adapt them for use as protective coatings and for structural applications. Quite obviously superior corrosion resistance is oifered at elevated temperatures and with melting points as indicated. Forming is not a problem. The alloys can be made up in the manner indicated above or by other conventional processes and then formed.

I claim:

1. An alloy characterized by its ability to withstand high temperatures and consisting of chromium from about 30% to about 48%, titanium from about 30% to about 48%, and tin from about 4% to about 8%.

2. An alloy characterized by its ability to withstand high temperatures and consisting of about 4 7 /2% chromium, about 47 /2 titanium, and about 5% tin.

3,265,491 5 6 3. An alloy characterized by its ability to withstand high temperatures and consisting of about 46% chromium, high temperatures and consisting of from about 36% 'to a130111467?) titanium, and a901K870 Zlfcomumabout 56% chromium, from about 36% to about 56% References Cited by the Examiner titanium, and from about 1% to 15% zirconium. r

4. An alloy characterized by its ability to withstand a FOREIGN PATENTS high temperatures and consisting of from about 40% to 594,658 3/1960 Canada.

about 50% chromium, from about 40% to about 50% titanium and from about 4% to about 12% zirconium. DAVID RECK Primary Exammer' 5. An alloy characterized by its ability to withstand 10 'N- L Examine"- 

1. AN ALLOY CHARACTERIZED BY ITS ABILITY TO WITHSTAND HIGH TEMPERATURES AND CONSISTING OF CHROMIUM FROM ABOUT 30% TO ABOUT 48%, TITANIUM FROM ABOUT 30% TO ABOUT 48%, AND TIN FROM ABOUT 4% TO ABOUT 8%. 